Battered Reject Balls and New Spherical Balls Compared: Operators are sometimes at a loss to know when a mill should be dumped to discard the worn and battered balls. Similarly, the present authors found an unknown variable when they compared the laboratory work of a new grinding medium with the plant performance of media, some of which may have been in the mill so long that they no longer retained their original shape. Accordingly, reject balls that had assumed the shape of polyhedrons were obtained and compared in the laboratory with new balls of the same weight. The results are shown in table 24. Both the power and the time of grind were greater for the rejects (the efficiency of the rejects was about 11 percent less than with the new spherical balls).